What is the best thing to do when my iPhone or iPad damages?

I went upstairs yesterday giving my nephew (he's 26 years younger than me or age 7) his iPhone 4 since he needs it, but it slipped off my hand, it fell down to the 1st floor, and it cracked. He actually owns an iPhone 4, should I not fix it to not waste money, recycle it, fix it at a third party (Talk N. Fix or any other cell phone or tablet repair, or replace the iPhone 4 for an iPhone 6s at the Apple Store)? He got a new iPhone 4 because when I attempt to fix his iPhone 3GS in 2013 (or the 3rd party), it glitches, and it started touching random places, and clicks on random things, the screen is unresponsive (it is sold on Ebay since Amazon doesn't accept it). If I don't fix it, or if I hid it, will it have a higher chance of dying? And, is replacing an iPhone 4 better than fixing it at the Talk N. Fix or any other place? (It costs $100).